A rock anchor is known from earlier U.S. Pat. No. 4,906,149 which comprises a sleeve formed of ST34 or ST37 plastically deformable steel and having an outer end formed as an enlarged head so that the sleeve can be inserted in a bore in a rocky mass with the head lying against the face of the rock. At its inner end the sleeve is provided with a hard wedge having a flank or face extending at a sharp acute angle to the longitudinal axis of the sleeve. A spreading element is engageable between this inclined flank and the adjacent inside surface of the sleeve, and a drive pin can be hammered into the sleeve to push the spreading element in. This action forces the wedge back in the sleeve and simultaneously jams the spreading element between the wedge and the inside sleeve surface to plastically deform the sleeve outward, thereby solidly locking it in the rock hole. Such an anchor can withstand withdrawal forces of 10,000N (1000 kp) in a hard rock or concrete wall.
For best results the bore in which the anchor is set must be equal to or at most only slightly-longer than the length of the sleeve from the inside face of its head to its inner end. When, however, the hole is made too deep, something that is relatively common when boring with a big hammer drill, the hold that the anchor will attain will be substantially diminished because the wedge will be driven at least partially out the back or inner end of the sleeve. It is even possible for the hole to be so long that the wedge is driven completely out of the inner end of the sleeve, so that there is no deformation at all of the sleeve and the anchor is useless.